This invention relates generally to methods and apparatus for applying uniform coatings to a moving web of paper, and more particularly to a method and apparatus for coating a moving web using a coater having multiple extraction ports in the application zone between a static converging wedge and a dynamic converging wedge that interact to control the amount of liquid coating material applied to moving web and provide regional excess coating liquid and air content removal to better control coating uniformity in a cross-machine direction. An optional coating sensor can be used downstream from a final metering element to further control coat-weight profiles by adjusting the removal rate locally.
In some known methods for controlling a profile of coating on a backing roll or a substrate, a flexible metering element is positioned with its edge adjacent to the substrate to remove excess coating on the substrate. Pressurized tubes or single-profile solid bars have been used to apply a biasing force to the flexible blade or other metering element such as a rod or plate to counter the dynamic forces of liquid coating moving along with the surface of the web to be coated. In this manner, the metering element will control the coat-weight profile in a cross-machine direction. However, coat-weight non-uniformity will occur during the coating process due to varying flow conditions inside the coating applicator, changing coating rheologies, web speeds, metering blade wear, and non-uniform backing roll wear. The conventional approach to regain uniformity has been to increase or decrease localized pressure against the metering element to minimize high or low-coat weight regions. Although providing satisfactory coating profiles, the metering element and the backing roll experience accelerated wear, particularly in the local regions of higher pressure. Rigid metering elements also have been used to control coating profiles. Rigid metering elements such as a rod or plate, are uniformly adjustable along their length using air tubes or hydraulic actuators. With rigid metering elements, adjusting coating profile in a cross-machine direction is limited and, in some instances, not possible.
Therefore, it is desirable to have a method for using a coating applicator to adjust coat-weight profiles in a cross-machine direction at high machine speeds and without excessive metering element wear or backing role wear.